Gas, O (6)
I believe the answer is:
oxygen
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">gas</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that oxygen is a type of gas)</span><br/ ><br/ >'<span class="explanation-format__original">o</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(O is the chemical symbol for oxygen)</span><br/ ><br/ ></span>
(Other definitions for oxygen that I've seen before include "Atmospheric gas" , "Life supporting element" , "Second commonest element in air" , "Gas forming about 20 per cent of atmosphere" , "Most abundant element in the earth's crust" .)